1. Field of the Invention
The present invention generally relates to a lithium titanate and a method of forming the lithium titanate. More specifically, the present invention relates to a lithium titanate that has excellent electronic conductivity and excellent electric power-generating capacity.
2. Description of the Related Art
Motor vehicles such as, for example, hybrid vehicles use multiple propulsion systems to provide motive power. The most common hybrid vehicles are gasoline-electric hybrid vehicles, which include both an internal-combustion engine (ICE) and an electric motor. The gasoline-electric hybrid vehicles use gasoline to power the ICE, and an electric battery to power the electric motor. The gasoline-electric hybrid vehicles recharge their batteries by capturing kinetic energy. The kinetic energy may be provided via regenerative braking or, when cruising or idling, from the output of the ICE. This contrasts with pure electric vehicles, which use batteries charged by an external source such as a power grid or a range extending trailer.
The batteries include rechargeable lithium-based cells that typically comprise two dissimilar electrodes, i.e., an anode and a cathode, that are immersed in an ion conducting electrolyte, with a separator positioned between the two electrodes. Electrical energy is produced in the cells by an electrochemical reaction that occurs between the two dissimilar electrodes.
The largest demand placed on the battery occurs when it must supply current to operate the electric motor during acceleration, especially during start-up of the electric motor. The amperage requirements of the electric motor may be over several hundred amps. Most types of batteries that are capable of supplying the necessary amperage have a large volume or require bulky packaging, which results in excessive weight of the batteries and adds cost to the batteries. At the same time, such high currents are only required for short periods of time, usually seconds. Therefore, so called “high-rate” batteries, which provide high currents for short periods of time, are typically ideal for hybrid and pure electric vehicle applications.
Rechargeable batteries that include rechargeable lithium-based cells, which may be characterized as either lithium cells, lithium ion cells, or lithium polymer cells, combine high electric power-generating capacity with the potential for power and cycle-life needed to enable the hybrid vehicles to meet performance standards while remaining economical. By “high electric power-generating capacity”, it is meant that the rechargeable batteries have four times the energy density of lead-acid batteries and two to three times the energy density of nickel-cadmium and nickel-metal hydride batteries. Rechargeable batteries including the lithium-based cells also have the potential to be one of the lowest-cost battery systems.
Lithium titanate represented by the formula Li4Ti5O12 (or Li4/3Ti5/3O4) is considered to be one of the most prospective materials for use in the anodes of rechargeable lithium ion and lithium polymer cells. Lithium titanate, Li4Ti5O12, is known from A. Deschanvers et al. (Mater. Res. Bull., v. 6, 1971, p. 699). As it was later published by K. M. Colbow et al. (J. of Power Sources, v. 26, N. 3/4, May 16, 1989, pp. 397-402), Li4Ti5O12 is able to act in a reversible electrochemical reaction, while elemental lithium is incapable of such reversible reactions. After detailed research conducted by T. Ozhuku et al. (J. of Electrochemical Society, v. 142, N. 5, 1995, pp. 1431-1435) the Li4Ti5O12 started to become considered for use as an anode material for rocking-chair type lithium cells. In fact, U.S. Pat. No. 5,545,468 to Koshiba et al. discloses the use of a lithium titanate having varying ratios of lithium to titanium in the lithium titanate. More specifically, the lithium titanate of the '468 patent is of the formula LixTiyO4, wherein 0.8≦x≦1.4 and 1.6≦y≦2.2, in a cathode for a lithium cell. The '468 patent specifies that fundamentally, x+y≈3. In other words, the '468 patent teaches that the lithium titanate may include different ratios of lithium to titanium, so long as the amount of lithium and titanium together about 3 such that there is a stoichiometric amount of lithium and titanium to oxygen. United States Patent Publication No. 2002/0197532 to Thackeray et al. also discloses a lithium titanate that is used as an anode in a lithium cell. The lithium titanate may be a stoichiometric or defect spinel, in which the distribution of lithium can vary from compound to compound.
In addition to an ability to act in the reversible electrochemical reaction, Li4Ti5O12 also has other advantages that make it useful in rechargeable lithium-based cells. For example, due to a unique low volume change of the lithium titanate during the charge and discharge processes, the lithium titanate has excellent cycleability, i.e., many cycles of charging and discharging may occur without deterioration of the cells. The excellent cycleability of the lithium titanate is primarily due to a cubic spinel structure of Li4Ti5O12. According to data of S. Scharner et al. (J. of Electrochemical Society, v. 146, N. 3, 1999, pp. 857-861) a lattice parameter of the cubic spinel structure (cubic, Sp. gr. Fd-3m (227)) varies from 8.3595 to 8.3538 Å for extreme states during charging and discharging. This linear parameter change is equal to a volume change of about 0.2%. Li4Ti5O12 has an electrochemical potential versus elemental lithium of about 1.55 V and can be intercalated with lithium to produce an intercalated lithium titanate represented by the formula Li7Ti5O12, which has a theoretical electric power-generating capacity of up to and including 175 mA*hrs/g.
Another advantage of Li4Ti5O12 is that it has a flat discharge curve. More specifically, the charge and discharge processes of Li4Ti5O12 take place in a two-phase system. Li4Ti5O12 has a spinel structure and, during charging, transforms into Li7Ti5O12, which has an ordered rock-salt type structure. As a result, electric potential during the charge and discharge processes is determined by electrochemical equilibrium of the Li4Ti5O12/Li7Ti5O12 pair, and is not dependant on lithium concentration. This is in contrast to the discharge curve of most other electrode materials for lithium power sources, which maintain their structure during the charge and discharge processes. For example, although a transition of a charged phase in most cathode materials such as LiCoO2 is pre-determined, there is still an extended limit of variable composition LixCoO2 between these structures. As a result, electrical potential of materials such as LiCoO2 depends on a lithium concentration in the LiCoO2, i.e., a state of charge or discharge. Thus, a discharge curve in materials in which the electrical potential is dependent on the lithium concentration in the material is typically inclined and is often a step-like curve.
There is a general consensus within the art that maintenance of excellent electric power-generating capacity correlates to excellent electronic conductivity. Li4Ti5O12 includes titanium in a highest oxidation degree of +4, which correlates to very low electronic conductivity. An electronic conductivity of similar compounds is so low that many of those compounds are borderline dielectrics or insulators. As such, power generating capacity of Li4Ti5O12 is less than ideal. The same holds true for the lithium titanates of the '468 patent and the '532 publication, as set forth above.
Typically, electronic conductivity of the Li4Ti5O12 is improved by doping the Li4Ti5O12 with 3d-elements, as disclosed by M. Nakayama et al (Solid State Ionics, v. 117, I. 3-4, 2 Feb. 1999, pp. 265-271). For example, electronic conductivity of Li[Li(1−x)/3CrxTi(5−2x)/3]O4, which is considered to be a solid solution between Li4Ti5O12 and LiCrTiO4, is better than electronic conductivity of the Li4Ti5O12. However, an increase in the amount of Cr ions substituted for titanium ions in the Li4Ti5O12 also decreases reversible electric power-generating capacity, as compared to Li4Ti5O12, due to electrochemical inactivity attributable to the presence of the Cr ions. The presence of the Cr ions lowers area specific impedance (ASI) and increases rate capability, as compared to ASI and rate capability of Li4Ti5O12. The loss in capacity is substantially equal to the share of replaced titanium.
Other attempts to replace the titanium in lithium titanates exhibit similar drawbacks. For example, substitution of titanium in Li4Ti5O12 with vanadium, manganese, and iron results in significant loss of reversible electric power-generating capacity during a first charge-discharge cycle. See P. Kubiak, A. Garsia, M. Womes, L. Aldon, J. Olivier-Fourcade, P.-E. Lippens, J.-C. Jumas “Phase transition in the spinel Li4Ti5O12 induced by lithium insertion. Influence of the substitution Ti/V, Ti/Mn, Ti/Fe” (J. of Power Sources, v. 119-121, Jun. 1, 2003, pp. 626-630).
In view of the foregoing, there remains an opportunity to provide a lithium titanate that is modified to exhibit excellent electronic conductivity while maintaining reversible electric power-generating capacity that is characteristic of lithium titanate. There is also an opportunity to provide a lithium-based cell that includes the lithium titanate.